The genetics of herpes simplex virus type 1 (HSV-1) is being examined using cloned HSV sequences. The initial portion of these studies has established the feasibility of introducing cloned HSV sequences into infectious virus by cotransfection. We are presently studying novel HSV recombinants which we have generated in alternate vectors (simian virus 40 (SV40) and bacteriophage lambda 1059). These cloned sequences will be available for mutagenesis and studies of gene expression in the latter course of our experiments.